The invention relates to a method for determining the pH of liquids by a pH difference measurement on a solution under test, and also to an apparatus for determining the pH of liquids.
It is presently possible to measure absolute pH of liquids with a precision of about 0.01 pH. This applies particularly in measurements of alkaline solutions since the glass electrodes used in the measurement have an alkali error of 0.05 pH, for example in a 0.1N NaOH solution, which cannot be calibrated.
In the prior art processes, the pH is determined, for example, with the aid of hydrogen electrodes (Grundlagen der physikalischen Chemie (Principles of physical chemistry), 4th edition, 1963, author: Brdicka, page 626). In this process, the potential of a hydrogen electrode in a solution having any pH value is determined with the aid of a reference electrode. In order to reduce to a minimum the diffusion potential at the interface of diverse electrolytes, the electrode solutions are separated by a bridge. The reference electrode used is, for example, a saturated calomel electrode. If solutions of different concentrations are in contact through a porous wall or a permeable membrane or directly in contact, an electrical double layer forms at their interface, so that a potential drop is produced between the two solutions. This electrical double layer forms as a consequence of the diffusion of the electrolyte out of the concentrated solution into the dilute solution. If the cations and anions of the respective electrolyte have a different mobility, the more mobile type of ion tends to advance quickly in the diffusion. However, the electrostatic forces acting between the oppositely charged ions prevent the independent diffusion of both types of ions and, consequently, the more mobile ions are retarded in their diffusion by the less mobile ions, and, conversely, the diffusion of the slower ions is accelerated by the more mobile ions. The tendency of the more mobile ions to diffuse more rapidly is manifested in the fact that said more mobile ions form a layer of similar charges which is immediately followed by a layer composed of the slower ions with opposite charge, which results in the formation of the electrical double layer. This produces a potential change at the liquid interface. In general, concentration chains having such a potential change between solutions in contact are described as concentration chains with transfer (Grundlagen der physikalischen Chemie (Principles of physical chemistry), 4th edition, Brdicka, page 612).
In determining the electromotive force of concentration chains, the diffusion potential at the interface of the solutions of different concentrations can be suppressed by connecting the electrode solutions by means of a saturated solution of potassium chloride or ammonium chloride. Such a connection is described as a bridge, and instead of one interface between the electrolytes, two interfaces are produced resulting in substantial elimination of the potential drop between the electrolytes.
Calomel electrodes are used as reference electrodes for determining the potential of other electrodes because of their completely reproducible potential. For practical measurement purposes, depending on the requirement, the calomel electrodes are filled with either a 0.1N, 1N or saturated potassium chloride solution as electrolyte bridge.
Using the calibrated reference electrodes or calomel electrodes and a pH-sensitive electrode, for example, a glass electrode, the absolute pH of the electrolytes can be determined by comparison with buffered standard solutions.
In many cases, however, a pH difference measurement in which the solution to be examined is compared with a known or arbitrarily chosen reference solution as standard is sufficient.